While in Switzerland I found some foam rubber sheets (I guess that's how you call them- in German the material is called Moosgummi). I cut them into shapes and the girls could "glue" them with some water to the window. I saw this idea on another blog but just can't remember where exactly. It is a very nice activity and the shapes are also easy to take down and they leave no marks. You could also use them on the bathroom tiles for some fun during bath-time.